1. Technical Field
This invention relates generally to signal processing and more particularly to an improved system for providing precision control of single phase or three phase AC line power supplied through thyristor type switching devices to a load such as a resistance spot welding machine.
2. Background Art
Resistance spot welding is a process used to join metals, such as steel, aluminum, titanium and metal-matrix composites, in which coalescence is produced by the heat generated when electric current passes through resistive work pieces held together by electrodes. This process is widely used in aerospace, automotive, appliance and electronic component manufacturing.
Although the resistance welding process has been in existence since Elihu Thompson invented it in 1877, the assurance of weld quality remains a serious concern. Critical to the production of good welds is the ability of the welding machine to reliably deliver actual percent heats in accordance with a desired weld schedule. The term "percent heat" refers to a percentage of the maximum heat that a particular type of welding machine and workpiece configuration may produce. In order to avoid expulsion, i.e. the spewing out of molten material during the welding process, the rate of heat application to a particular type of work piece must be controlled. The heat setting(s) for various cycles of the welding operation is referred to as the weld schedule. A suitable weld schedule for a particular application is typically determined through trial and error and may comprise either a single heat setting or heat settings that vary cycle by cycle or power pulse by power pulse. Once the weld schedule has been determined, it is important that the actual percent heat produced during the welding process closely conform thereto in order to assure consistent, high quality welds.
In resistance spot welding machines, silicon controlled rectifiers (SCRs) or other thyristor type switching devices connected to the primary of a welding transformer control the line power transmitted to the weld electrodes at the secondary of the welding transformer. A pair of back-to-back thyristor type devices are provided at the primary for each input phase signal and are selectively switched on by triggering pulses to transmit power pulses to the weld electrodes. The triggering times are chosen with the goal of providing the desired percent heat to the work piece.
In prior art digitally based thyristor type power controllers, the trigger time of the next thyristor to conduct must be specified prior to the previous zero crossing of the corresponding power line input. This prior art method of digitally controlling thyristor firing times, in the case of a three phase DC power control, involves the utilization of a separate programmable delay for each phase to control the SCR trigger times on that phase. FIG. 9 demonstrates this architecture. The trigger time on each phase is timed from the previous zero crossing of that phase. The desired programmed delay time must be loaded into the programmable delay element (typically a down counter) by the computer prior to this time.
For three phase full wave operation, SCR trigger times must occur between 60.degree. and 180.degree. past the zero crossing. Accordingly for a 60 Herz line frequency, this means that the computer has to have loaded the desired programmed delay time into the programmable delay elements between 2778 microseconds and 8333 microseconds prior to the actual triggering. A power control system which would allow the desired programmed value to be loaded later than this time, and in fact as late as theoretically possible, would be highly desirable since thousands of additional computer operations could be performed during this additional time.
Three phase power control is more complicated than single phase power control and cannot be effectively implemented by merely providing three redundant single phase controllers. In single phase, the thyristor which turns off when its current becomes zero, always turns off naturally at the zero crossing. However, in three phase, a power pulse can be terminated prematurely due to the firing of a subsequent power pulse whose instantaneous voltage is a higher value than that of the present power pulse. This phenomenon which is referred to herein as "commutation" results in the maximum heat per power pulse deliverable by a three phase welding machine being less than that of a single phase machine having a primary which is electrically comparable to a primary of one phase of the three phase machine input. Prior three phase power controllers which fail to recognize and accommodate this phenomenon do not provide linear scaling of "percent heat" with respect to the actual maximum heat deliverable by the machine/workpiece configuration.
Variations in input line power to a welding machine may cause the actual percent heat produced to differ from the desired percent heat. There are several types of prior art primary line monitoring systems. One approach, exemplified by U.S. Pat. No. 4,499,363, monitors the current in the primary. This current monitoring and regulating approach is suitable for arc welding but inapplicable to resistance spot welding since the feedback it provides is not independent of work piece variations at the secondary and in fact provides a contra-indication of steps which need be taken to compensate for input line variations.
A second basic prior art approach is to monitor primary line voltage. Such prior art line voltage monitoring systems generally fall into two categories: (1) those which monitor the average line input value of a single phase, and (2) those which monitor the average value of a three phase full wave rectifier on the line inputs. In both these cases the line input voltage is measured and not the actual voltage input applied by the thyristor type devices to the welding machine. Accordingly, input line power variations which may have no impact on the welding operation because they are not transmitted to the weld electrodes may be unnecessarily detected and compensated for. Further, in case 1, variations on the other phases are not taken into account. In case 2, the signal monitored is representative of the weld machine input when 100 percent heat is applied only. For lower percent heat settings the monitoring system of case 2 does not accurately represent the weld machine input. These prior art systems multiply the measured percent line variation by the expected heat setting to obtain an approximation of the signal error. They are unable to monitor the actual percent heat produced by each individual power pulse. Without such actual pulse-by-pulse measurements it is impossible to accurately compensate in "real time" for those variations in input power which are transmitted to the welding machine.
A need thus exists for a programmable power control and line voltage monitoring and compensation system which affords precise control over actual percent heat delivered by resistance spot welding machines of all configurations, maximizes the time available for determining SCR trigger times, takes into account the commutation effect in three phase welding machine configurations, and in "real time" and at any percent heat setting accurately monitors and compensates for only those input line variations which are transmitted to the weld machine.